The present invention relates to an insulating material for use on refractory articles.
1. Background of the Invention
Insulating refractory materials are used in industry, notably in metallurgy, for reducing heat losses and for saving energy. These materials can also be used for coating an article. They can also be used for producing an insulating component in the material itself. They can also be used to produce elements such as panels or bricks that will be used severally to form an insulating unit.
For example, refractory material components are used in the continuous casting of steel for the transfer of molten steel between various containers. notably between the ladle and the distributor, and the continuous casting mold. These components should be made more insulating thermally to improve the preheating efficiency (when the components are preheated), to avoid solidification of the steel an the inside walls of the taphole and, in the case of pouring components used for a distributor, to avoid bridging between the casting component and the mold walls.
According to another example, to improve the preheating efficiency of the distributor stopper rods, a sleeve of insulating refractory material that is fitted on the stopper rod and retains the heat of the burner is used.
It is customary to use sheets or mats of paper impregnated with ceramic fibers as the insulating refractory material. This material guarantees a good quality of the thermal insulation, but presents several shortcomings.
The placement of the ceramic paper requires operations of cutting, placement and gluing that are long and tedious. Furthermore, the handling of sheets or plates of ceramic paper permits the escape of carcinogenic ceramic fibers that can be inhaled by the operators.
An insulating refractory coating for steel casting components is also known (EP 0 296 981). This coating is obtained from a composition of an aqueous suspension containing 30-85% by weight of a finely divided constituent such as fused silica, alumina or zirconia powders and alumina beads, a ceramic charge of fibers of alumina silica, zirconia, titanium dioxide or chrome-alumina, or even alumina or zirconia beads. This composition is also comprised of up to 7% by weight of a binder such as sodium hexametaphosphate or sodium silicate, and up to 40% by weight of glass generating frit.
Such a coating avoids some of the disadvantages of plates of ceramic paper impregnated with ceramic fibers. In particular, it is more rapid to install because it does not require a large number of operations. In addition, it can facilitate avoiding the presence of ceramic fibers that are dangerous for the health of the operators. However, it presents certain disadvantages. Its thermal insulation characteristics are not very good due to its low porosity (ca. 20%). This porosity is essentially open, which assures less good thermal insulation characteristics than a closed porosity. Furthermore, it is difficult to deposit a substantial thickness of coating on a casting. To increase the thickness of the coating, it is necessary to preheat the components before coating them, which requires a supplementary stage and implies a supplementary cost. Furthermore, after having deposited a first layer, it is not possible to deposit a second layer because the outer surface of the coating is smooth and impermeable, which does not permit a good adhesion of the second layer.
2. Description of the Invention
The object of the present invention is an insulating refractory material that remedies these shortcomings. This insulating refractory material is comprised of 20-80% by weight of a ceramic matrix, 5-40% by weight of insulating microspheres, 0.5-15% by weight of one or more binders and up to 5% by weight of water. The matrix can be a matrix of vitreous grains, notably silica, preferably atomized silica; it may also comprises non-vitreous grains such as alumina or magnesia. Preferably, the matrix does not comprise more than 30% by weight of the matrix of non-vitreous grains.
U.S. Pat. No. 4,874,726 discloses a refractory material having a high abrasion resistance and a relatively low thermal conductivity. This material comprises from 40 to 95% by weight of vitreous silica, up to 25% by weight of a calcined refractory aggregate and the balance of a calcium aluminate cement (binder). For each 100 parts of this mix, the material further comprises from 3 to 15% by weight of alumina-silica based microspheres. This composition intended for the formation of refractory pieces by cast in or gunning, has a viscosity, when mixed with the appropriate amount of water, of about 4 to about 6 Pa.s. This extremely low viscosity, usual for the formation of refractory pieces by casting operations, is not compatible with the formation of a coating by immersion.
The material can also have up to 4% by weight of a deflocculant and up to 20% by weight of colloidal silica. Preferably, it has from to 0.5 to 4% by weight of a deflocculant and from 0.5 to 20% by weight of colloidal silica.
The insulating microspheres may be hollow spheres of a material based on silica and alumina. This material contains 55-65% by weight of silica and 27-33% by weight of alumina.
The main characteristic of the binder of the invention is that it must confers to the aqueous composition (i.e. the slip) used in making the refractory material of the invention, a viscosity compatible with the formation of a coating by dipping or immersing the refractory piece into said slip. The formation of a regular and homogeneous coating depends indeed largely upon the viscosity of the slip. A suitable viscosity is generally higher than 8 Pa.s and preferably higher than 10 Pa.s.
Binders that might be used according to the invention are clays of the kaolinite type and organic binders such as the polysaccharides (e.g., dextrine).
These binders confer to the aqueous composition a viscosity which perfectly fulfills the above discussed requirements.
The invention also concerns a component, notably for casting steel, having a body of refractory material coated with the insulating material of the invention. The component can also be a composite piece produced partially of the material of the invention. This component can be produced, e.g., cast, in a single operation or formed of several assembled pieces.
The invention also concerns a process for preparing a composition for effecting an insulating coating or making an insulating piece.
According to this process:
one or more binders are dissolved in a quantity of water;
a deflocculant is added;
grains of atomized vitreous silica are added while agitating the solution to hydrate them and form a slip;
microspheres of an insulating material are added while continuing to agitate the slip to keep it homogeneous.
In a preferred variant of the process, colloidal silica is added after the deflocculant. The composition used in making the material can be a slip containing from 20 to 70% by weight of atomized vitreous silica grains, from 5 to 40% by weight of insulating microspheres, from 0.5 to 20% by weight of one or more binders and from 5 to 25% by weight of water. It may further comprises up to 4% by weight of a deflocculant and up to 10% by weight of colloidal silica. Preferably, it further comprises from 0.5 to 4% by weight of a deflocculant and from 0.5 to 10% by weight of colloidal silica. Such a composition has a viscosity of between 9 to 12 Pas.
The invention also concerns a refractory component having an insulating coating.
The invention also concerns a process for coating a piece of refractory material used in the casting of molten metal, particularly steel with the composition of the invention.
According to this process:
the piece is dipped at ambient temperature in a composition as described above for a time less than one minute;
it is allowed to dry in the open air for 2-4 hours.
It is possible to produce several layers with the aid of this process.
a first layer of slip is deposited on the piece by the first immersion;
the piece is allowed to dry in the open air for 45 minutes to 2 hours;
a second layer of slip is deposited on the piece by the second immersion;
the piece is allowed to dry in the open air for 2-4 hours.
The material of the invention presents numerous advantages.
It is easier to apply than the papers and mats of ceramic fibers. Nor does it generate fibers dangerous for health.
With regard to the coatings previously known, it assures a better thermal insulation because its porosity is greater and because the portion of closed porosity, furnished by the insulating microspheres, is more substantial. Furthermore, thanks to the particular viscosity of the slip, the quality of the thermal insulation is improved because it is possible to deposit a greater coating thickness on the piece without impairing the insulation characteristics. This thickness can range up to 4 mm in a single layer and up to 7 mm in two layers.